Polyether polyols are integral intermediate components utilized to manufacture a wide array of products, including polyurethanes. As such, the production of polyether polyols is critical. It is known in the art that polyether polyols are produced from the polymerization of epoxides, such as ethylene oxide (EO) and propylene oxide (PO). It is also known in the art that double metal cyanide (DMC) catalysts are effective catalysts for the polymerization of the epoxides. DMC catalysts produce polyether polyols having narrow molecular weight distributions as well as relatively low unsaturation.
In conventional methods, DMC catalysts are synthesized by combining an aqueous solution of a metal salt and an aqueous solution of a complex metal cyanide salt. As a specific example, an aqueous solution of ZnCl2 (excess), as the metal salt, is combined with an aqueous solution of K3Co(CN)6]2, as the complex metal cyanide salt. This combination precipitates out the desired DMC catalyst, in this case specifically Zn3[Co(CN)6]2. Examples of such conventional methods are disclosed in U.S. Pat. Nos. 5,470,813 and 5,714,639. These conventional methods, in one form or another, utilize a complex metal cyanide salt. The complex metal cyanide salts are very expensive which limits the economic viability of utilizing DMC catalysts in the production of polyether polyols.
In other conventional methods, DMC catalysts are synthesized by combining an aqueous solution of a first metal salt with an aqueous solution of a second metal salt and with an aqueous solution of an alkali metal cyanide in a single step to synthesize the DMC catalyst. Examples of such conventional methods are disclosed in U.S. Pat. Nos. 6,436,867 and 6,593,268. Although such methods do not rely on use of a complex metal cyanide salt, they are remain deficient because the various reagents, i.e., the first metal salt, the second metal salt, and the alkali metal cyanide, are required to be soluble in water. This requirement constrains the choice of reagents and, therefore, the content and morphology of the resulting DMC catalysts. Furthermore, synthesis of DMC catalysts that relies on aqueous solutions results in DMC catalysts that have residual aqueous, i.e., water, based moisture which is detrimental to the DMC catalysts. In other words, such moisture ‘poisons’ the DMC catalysts. As such, with DMC catalysts synthesized with aqueous solution, further processing (such as stripping, drying, etc.) is required to remove the aqueous-based moisture before these DMC catalysts can be used to polymerize alkylene oxides to produce polyether polyols.
Due to the deficiencies of conventional methods for synthesizing DMC catalysts, including those described above, it would be desirable to provide a method of synthesizing DMC catalysts that does not utilize expensive complex metal cyanide salts as intermediates thereby improving the economic viability of DMC catalysts utilized in the production of polyether polyols. It would also be desirable to provide a method that does not rely on aqueous solutions of the various reagents used during the synthesis of the DMC catalysts such that the content and morphology of the DMC catalysts can be varied and controlled beyond conventional DMC catalysts.